1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrographic copying machine, a printer apparatus and a facsimile apparatus of a type in which after an electrostatic latent image is formed on a charged surface of a latent image carrier such as a photoreceptor drum, the electrostatic latent image is developed into a toner image, and more particularly, to the correction of rise of the surface potential of the latent image carrier in the beginning of image formation.
2. Description of the Prior Art
Generally, in an electrographic copying machine there is provided, for example, a photoreceptor drum rotating at a constant speed, and along the rotation direction of the drum, a charging section, an exposing section, a developing section, a transferring section, a cleaning section and a charge removing section are arranged. First, the drum surface is charged at the charging section, then, the drum rotates, and when the charged drum surface passes the exposing section, light reflected in the scanning of the original exposes the drum surface. By the exposure light, an electrostatic latent image is formed on the drum surface.
When the drum rotates to the developing section, toner supplied from a developer unit arranged to face the drum surface adheres to the electrostatic latent image on the drum surface, so that a toner image is obtained. The toner image is transferred at the transferring section to the surface of a sheet supplied from a paper feeding section. After the transfer, residual toner on the drum surface is removed at the cleaning section, and the electrostatic latent image on the drum surface is removed by irradiating charge removing light to the entire drum surface at the charge removing section to optically attenuate the surface potential of the drum surface.
In the electrographic copying machine of the above-described arrangement, a charger employing a corona discharge method is arranged in the charging section to face the drum surface. At the time of the charging, a charge is supplied to the drum surface by applying high voltage of approximately 4 to 6 kV to a discharging main wire of the charger to generate a corona discharge. According to a conventional method, to supply the high voltage to the main wire, a transformer board having a transformer for generating a high voltage is provided between the main wire and a power source, and the transformer board is controlled so that its output is substantially constant.
According to such a charging method, the rise of the surface potential in the beginning of copying differs depending on the type of photosensitive material formed on the surface of the photoreceptor drum. Specifically, as shown in FIG. 7, when a copy button is pressed to start a copying operation in the waiting state of the copying machine, the high voltage is applied to the charger as described above, so that a charge is supplied to the drum surface. At this time, when arsenic selenium is used as the photosensitive material, the rise of the surface potential is made as shown by the broken line a of FIG. 7 such that the potential overshoots to temporarily exceed a stable potential and then returns to the stable potential to remain stable.
On the contrary, when an amorphous silicon is used which has been widely used as the photosensitive material of image forming apparatuses of this type in recent years, as shown by the solid line b of FIG. 7, it takes a long time for the potential to reach the stable potential after a copy button is pressed, i.e. the rise characteristic is inferior. In addition, it is known that with such a photosensitive material, the rise of the surface potential further deteriorates as shown by the solid line c of FIG. 7 according to the left period from the end of a copying process to the start of the next copying process.
In recent years, the time required for the first copying has been reduced to improve the copying efficiency. In a copying machine of a conventional arrangement where such an amorphous silicon material is used as the photosensitive material, as shown in FIG. 7, when the first copying is performed, the potential on the drum surface is still lower than the stable potential. For this reason, the charge amount of the electrostatic latent image is insufficient, so that an excellent toner image cannot be developed at the developing section. In addition, when the left period from the end of the copying process to the start of the next copying process exceeds one hour, the rise of the surface potential further deteriorates. For this reason, the charge amount of the electrostatic latent image is insufficient, so that an excellent toner image cannot be developed at the developing section.
Further, when continuous copying is performed in such an arrangement, the surface potential of the drum is low during the copying of the first and subsequent several sheets, and it is difficult to obtain a copy image of a desired quality before the copying of the several sheets are performed, i.e. before the drum surface potential reaches a normal value.